So far, that’s been only one direction: forward. Though listed as a center back against Thailand, Ertz spent most of the game in her more typical midfield spot, leaving Abby Dahlkemper behind to chat with goalkeeper Alyssa Naeher for the duration of the 13–0 rout. Ertz was back in the midfield against Chile, but with Morgan Brian and Lindsey Horan there to cover for her, she spent much of her playing time as far up the pitch as you’ll ever see her, like a rec league defender abandoning positional constraints in pursuit of a rare goal.

But Ertz scores plenty of goals, including one against Chile in her usual fashion: meeting Tierna Davidson’s corner kick at the near post and planting an unstoppable header into Chile’s goal.

The goal captures every facet of Ertz’s game: her anticipation, her aggressiveness, her physicality. Watch how she overruns the ball’s path in order to bump her marker out of the way and then adjusts back in time to meet the cross. The header itself is essentially perfect. With one snap of her head, she redirects all the momentum from the cross, bulleting the ball into the near corner. (We know it was unstoppable because if it could have been stopped, Chilean goalkeeper Christiane Endler would have stopped it.)

Ertz will continue to be the Americans’ most dangerous target on set pieces, but from here on out, her tournament will be judged not by the goals she scores—the U.S. has plenty of people who can find the back of the net—but the ones she prevents. On Thursday, for the first time in this World Cup, she’ll be asked to perform in the role she’s on the team to fill.

There may be no more crucial spot in the modern game than the defensive midfield position. The U.S. hit its highest gear in 2015 when Morgan Brian entered the starting lineup during the knockout round. France won the 2018 Men’s World Cup because it had N’Golo Kanté, and every other team did not. The North Carolina Courage finished with the best record in the National Women’s Soccer League the past two seasons in large part because of defensive midfielder McCall Zerboni (who many were surprised didn’t make the final U.S. roster for 2019).

There are only a handful of teams in the tournament who could hope to force the U.S. onto its back foot for more than a couple of moments a game. Ertz is there to make sure that those couple of moments are as brief and painless as possible—to track down runners and intercept passes before they even seem dangerous to viewers at home.

The key question is where on the field she is going to do this job. The traditional way would be to station herself in front of the defense, where even if she doesn’t win the ball she can disrupt or slow down opposing attacks enough to allow the center backs to finish the play.

But Ertz’s power is matched only by her range. Her instincts are to hunt the ball and win it as early as she feels she can, and she always feels like she can. If she can regain possession near the opponent’s goal, then the U.S. can shift itself into attack mode that much sooner. If she can’t, then either Horan is going to have to spend most of the game covering for her or the U.S. defense is going to be left exposed.

Ertz’s dilemma is a microcosm of the challenge facing this U.S. team for the rest of the tournament: how aggressive to be. What is the proper level of pressure to apply to each opponent? Is it better to take the safe option, to hold position, get a stop, then trust your talented attackers to do something on the other end? Or should the objective be to suffocate every opponent like it did Thailand and Chile, win the ball as early and as high up as possible, and keep defenders constantly under siege?

The latter strategy uses Ertz to her full potential, but it might not be the best long-term plan for this American roster. As Ertz pushes forward to win the ball, the defense has to move up too to prevent dangerous pockets of space between the lines. But as they push up, they leave space for teams to counter, and neither of the U.S. center backs is at her most comfortable in a footrace.

The U.S. could get away with swarming Thailand and Chile when their midfielders had the ball. Now it needs to decide at what point to ease off and sacrifice pressure for structure. Can it get away with Ertz pursuing the ball deep into opposing territory against Sweden? Against France or Germany? Where Ertz goes, the rest of the U.S. will follow. How far will that be this World Cup?